1. Field of the Invention
The present invention relates to a laser processing apparatus and a laser processing method, in which liquid is applied to a surface of a substrate such as a semiconductor wafer or a glass substrate for use in liquid crystal displays, and the substrate covered with liquid is scanned with a laser beam and thereby processed.
2. Description of the Related Art
Methods of manufacturing semiconductor devices include steps of processing the surface of a substrate, by using a laser beam. Among these steps are: processing a thin film such as an insulating film, a metal film or a resist film; cutting dicing lines; removing resist films from alignment marks before exposing the substrate to light (i.e., step of exposing the alignment marks). It is desired that a laser beam be used in these steps because it has a high energy density and can accomplish high-precision alignment.
When a laser beam is applied to a substrate to process the surface of the substrate, some substance is removed from the substrate sticks, in the form of vapor. The substance sticks to the surface of the substrate. It is proposed that, to prevent the substance from sticking to the substrate, the surface of the substrate be covered with liquid and then be scanned with a laser beam. Jpn. Pat. Appln. KOKAI Publication No. 2003-249427, for example, discloses a laser processing apparatus. As FIG. 1 shows, the apparatus has a cup 11, a drive mechanism 12, and a chuck 13. The drive mechanism 12 can rotates the chuck 13 and move the chuck 13 in the X- and Y-directions, positioning the chuck 13 in the cup 11. The chuck 13 may hold a substrate 10, which has alignment marks and resist films covering the alignment marks. A laser beam 18 is applied to the resist films, removing the resist films from the substrate 10. A transparent plate 15 made of quartz glass is spaced a little from the substrate 10, facing the substrate 10. Pure water is kept flowing through gap between the substrate 10 and the transparent plate 15, supplied through an inlet port and drained from the gap trough an outlet port. After the substrate 10 has undergone the laser processing, the plate 15 is raised and rotated at high speed. By virtue of centrifugal force, the pure water is removed from the substrate 10, thus drying the substrate 10.
In the laser processing method using a liquid film, it is necessary to shape the laser beam 18 so that the beam may have an appropriate cross-sectional shape. Unless the beam is so shaped, a groove of an undesirable shape will be made in the surface of the substrate when the beam is applied to the substrate. As FIG. 2A shows, the pulse laser beams 160 may be sequentially applied, thus scanning a selected region of a substrate, in the direction of arrow 152, along a dicing line 150. In this case, great energy is abruptly applied over the entire width of the dicing line 150 if the pulse laser beams 160 have a rectangular cross section. Consequently, the resultant groove may not have sharp upper edges. When the substrate is cut along such grooves, chips are provided, each having but an undesirable shape.
To prevent this, the cross-sectional shape of the beam must be changed to, for example, a shape shown in FIG. 2B. This shape is defined a first rectangle and a second rectangle. The second rectangle adjoin the middle part of the first rectangle and positioned in front thereof in the scanning director. Thus, the laser beam needs to have a cross-sectional shape selected in accordance with the type of the process in which it is used, such as dicing process or thin-film removing process. An apparatus that changes the cross-sectional shape of a laser beam is described in, for example, Jpn. Pat. Appln. KOKAI Publication No. 2002-224878, paragraphs [0049] and [0148].
The inventors of this invention know that the cross-sectional shape of a laser beam should better be adjusted in accordance with the width and depth of the groove to be made by applying the laser beam, even if the groove is made in, for example, a thin film. In view of this, it is desirable to select a cross-sectional shape for the laser beam in accordance with the type of the process performed by using the laser beam. It is strongly demanded that an apparatus be provided, in which an appropriate cross-sectional shape can be easily set for the laser beam.
The apparatuses described in Jpn. Pat. Appln. KOKAI Publications Nos. 2003-249427 and 2002-224878 has no function of setting an appropriate cross-sectional shape for the laser beam. It is demanded that grooves of different widths be made in, for example, a thin film. To meet this demand, slits (patterns) must be changed to have widths corresponding to the widths of the grooves. This would take much labor and time.